This invention relates to circuitry which may utilize the resonance phenomenon to ignite and/or to operate a gas discharge lamp.
Gas discharge lamps, for example, fluorescent lamps, high pressure sodium lamps, neon signs, etc., usually require high voltages to fire. But, once they are ignited, the operating voltages are significantly lower. It is disclosed in above-mentioned U.S. Pat. No. 4,933,605 how a high frequency resonant inverter can very efficiently ignite and operate a gas discharge lamp.
A block diagram of a resonant inverter utilizing the integrated circuit (IC) SG2525 is shown in FIG. 1. The combination of CT2 and RT2 determines the oscillator frequency of the IC. A resistor R4 is usually required between the terminal 15 and 13. A resistor divider R5 and R6 determines the amount of DC voltage applied to non inverted terminal (pin 2) of the operational amplifier. This voltage, in turn, sets the magnitude of the duty cycle of the output pulses (pin 14 and pin 11). Depending on the requirement, an impedance Z2 is necessary between the inverted terminal (pin 1) and the compensation terminal (pin 9) of the error amplifier for loop stability of the IC.
Output signals from pin 11 and pin 14 periodically turn Q2 and Q3 on and off. Thus, when Q2 is on, Q3 is off, and when Q2 is off, Q3 is on. During the time when Q2 is on, energy flows through Q2 and the resonant inductor LR to charge the resonant capacitor CR. Then, when Q2 is off but Q3 is on, stored energy from CR flows back through LR and Q3. With this arrangement, if the pulse repetition frequency is identical with the resonance frequency of the LC (LR and CR) network, the circuit can be described as a resonant inverter.
One of the simplest, most efficient and economical ballast configurations based on a resonant converter technique is shown in FIG. 2.
In this case LR and CR form a resonant circuit and the lamp T1 acts like a load across CR. This is equivalent to the diagram of FIG. 3. The respective impedances of the circuit parameters of FIG. 3 can be described as follows: For the load, the impedance is RL, for the resonant capacitor, the impedance is 1/jw(CR)=-jXCR and for the resonant inductor, the impedance is jw(LR)=jXLR. Here, j is the complex number and w=2.pi.(fr)=x. fr is the excitation frequency. At resonance, XCR=XLR. Further, ##EQU1##
In the case of FIG. 3, under the resonance condition, the voltage across CR or RL depends on the quality or Q-factor of LR and CR, and value of RL. This is true because, at resonance, jXLR-jXCR=.phi., that is, the impedances offered by the inductor and the capacitor are mutually cancelled. In the present application, RL is replaced by the lamp T1. Initially, before the lamp T1 fires, it offers an infinite impedance (that is, no current flow therethrough) and as a result the voltage across CR or T1 (FIG. 2) continues to grow. However, once the voltage across T1 reaches the lamp firing potential, the lamp T1 fires and offers much lower impedance. At this instance, due to the lamp chracteristic, the voltage across T1 clamps down to the normal lamp operating potential and stays there. This is a very convenient and reliable mechanism for starting and operating a fluorescent lamp.
During the normal operation, the current through the resonant inductor LR is equal to the vector sum of the current through the resonant capacitor CR and the current through the load or the lamp T. This is true, because, during the normal operation the lamp T can be considered mostly a resistive load and, as a result, the current through the capacitor CR will have 90 degree phase difference, with respect to the lamp current. Thus, the current through LR, which is also the total circuit current, can be described as, ##EQU2## Further, during normal operation, the voltage across the resonant capacitor is the same as the voltage across the lamp, .sup.V lamp. Thereby, the current through CR is, .sup.i CR, running=.sup.V lamp/XCR. On the other Hand, during starting, before the lamp fires, the current through the capacitor CR is determined by the ratio of the lamp firing potential to the impedance of CR. That is, ##EQU3##
Moreover, during starting, .sup.i CR, firing equals the total load current, which is circulating between CR and LR through the power switches Q2 and Q3. For this reason, if the lamp firing potential is very high, depending on XCR, a very large amount of circulating current can flow through Q2 and Q3 before the lamp fires. This large circulating current during starting may exceed the maximum rated current through Q2 and Q3 and thereby, may destroy Q2 and Q3.